Currently, in China, only a minority of areas has just resolved a problem of electric power shortage, a majority of areas and big cities still face an urgent problem of shortage in electric power supply, and quality of power supplies cannot be ensured. Mains electricity cannot provide a clean and stable power supply that is needed by a sensitive electronic device, and a user is finally responsible for healthy and safe running of the device. Even in America and other western countries that have implemented electrification for a long time, quality of a power grid is far from reliability. Because of a problem in quality of a power grid, impact of various chance factors, and damages from natural disasters, negative phenomena of a power grid such as a voltage surge, electromagnetic noise, a sustained high voltage, and a sustained low voltage are also common in a developed country, and a long-term power outage may even occur.
An uninterruptible power system (UPS) is a device that can take the place of mains electricity to sustainably supply power during a power outage, includes an energy storage apparatus (battery), and continues to supply power during a fault or interruption of mains electricity, which ensures safety and reliability of power consumption of an user, and avoids losses caused by a fault of the mains electricity.
Generally, the UPS includes the following several parts: a rectifier (alternating current/direct current (AC/DC)), an inverter (DC/AC), a charger (CHG), an energy storage apparatus (battery), a bypass (STS), and a direct current bus (BUS). The direct current bus is an output of a rectifier AC/DC circuit, and is also an input voltage source of the inverter DC/AC. When a mains input of the UPS is normal, the mains input supplies power to a load using outputs of the rectifier (AC/DC) and the inverter (DC/AC). At the same time, a direct current bus charges a battery using a charger. When the mains input of the UPS is faulty, a switch apparatus switches to an input end of the battery, and the battery outputs, using DC/DC conversion, electric energy stored in the battery to a direct current bus, which ensures a stable direct current bus voltage. At the same time, the direct current bus supplies power to the load using DC/AC inversion conversion, which ensures a normal output.
During general use, by controlling the switch apparatus, the UPS may select mains electricity or a battery to supply power. However, during a switch between the mains electricity and the battery, a particular gap period without a mains electricity input or battery input exists in a switch process, and a time of the gap period is generally between several milliseconds to tens of milliseconds. Energy (1/2CU2, where C is capacitance and U is voltage) stored in a direct current bus capacitor may maintain an uninterruptible output voltage, and power of the load is all supplied by energy stored in the direct current bus. Because the time is long, and the load is large, a requirement for a capacitance value of the direct current bus is high, and a cost competitive edge of a volume of the UPS is compromised.